Sound pressure, also known as acoustic pressure, is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air, and the SI unit for sound pressure is the pascal (Pa). The sound pressure level is a logarithmic measure of the effective sound pressure of a sound related to a reference value. Sound pressure level is measured in decibels (dB), typically above a standard reference level in air of 20 μPa RMS, which is usually considered the threshold of human hearing. Thus, the units for sound pressure level, when measured over the standard reference level are “dB (SPL)”.
Multiple microphones have been used in conjunction. For example, a combination of two microphone capsules by frequency division has been used. Such an implementation may be motivated to achieve a very wide frequency response by using, for example, a large membrane capsule for low frequency and a smaller membrane for high frequency. The combined result typically then achieves a smoother and wider response than either of the two microphone capsules individually may achieve. Another motivation to combine such microphone capsules by frequency division is that when directional microphone capsules are used, the polar pattern in first order microphones is, by nature, difficult to achieve for full audio bandwidth. By combining a microphone capsule with a good polar pattern in low frequency with another microphone capsule having a good polar pattern in high frequency, a wide bandwidth polar pattern may be achieved.